Exchange in, and Control of, Peripheral Circulation Flashcards
Quick capillary overview
- Specialised for exchange
- Lots of them
- Thin walled - prevents a small diffusion barrier
- Small diameter - big surface area: vol ratio
What is the blood brain barrier an example of?
A tight junction where endothelial cells are pressed up really close together and stops anything from moving across.
What do majority of capillaries have?
A gap between the cells called a cleft
What do clefts allow?
Water and some dissolved solutes to pass across.
Then macromolecules, including proteins, can be transported across individual endothelial cells via this process called transcytosis.
What are three classes of capillaries?
- Continous - no pored or clefts eg blood brain barrier or clefts only eg muscle and most other cap.
- Fenestrated - clefts and pores eg intestines, kidney, specalsied for fluid exchange
- Discontionus - clefts and massive pores eg liver
How does exchange happen in the capillaries?
Diffusion - higher concentratuion of oxygen in capillaries and moves down concentration gradient to ECF and into cells
Features of capillary diffusion:
Self regulated
Non-saturable
Non-polar substances across the phspholipid membrane
Polar substances through clefs/pores
Carrier mediated transport system - glucose transporter in the brain
Apart from diffusion, how dod substances move in the capillary?
Bulk flow determined by Starlin’s forces
What are Starling’s Forces?
Starling forces are the physical forces that determine the movement of fluid between capillaries and tissue fluid.
The two major starling forces are hydrostatic pressure and oncotic pressure.
Bulk flow in capillaries explained
Blood is going to be flowing from arteriole, capillary and venule due to a higher hydrostatic pressure.
40 mmHg pushing blood from the arteriole into the capillary and hydrostatic pressure is going to push water through the clefts and pores in the endothelial cells.
Larger macromolecules, such as proteins, don’t fit through so they’re going to remain in the blood.
As we move along, more and more water is going to be being lost because it’s being pushed.
So therefore the concentration in the capillary is going to increase.
That will then build up osmotic pressure, also known as an oncotic pressure that’s then going to pull water back into the capillary.
How does fluid balance out following bulk flow?
Over the course of a day, you’ll lose about 20 l of fluid being pushed out by that hydrostatic pressure, but then about 17 l of fluid is then regained because it’s pulled back in by the osmotic or oncotic pressure.
If you do the simple arithmetic, you’re then left with 3L of fluid that drains into the lymphatic system.
How does fluid move from lymphatic capillaries to CVS?
From the lymphatic capillaries, fluid then drains into the lymph nodes. This is all the little dots you can see here.
From the lymph nodes, fluid drains into larger lymphatic vessels and it makes its way back up towards the heart where it drains into the vena cava and that fluid is returned to the cardiovascular system.
How does fluid move from lymphatic capillaries to CVS?
From the lymphatic capillaries, fluid then drains into the lymph nodes. This is all the little dots you can see here.
From the lymph nodes, fluid drains into larger lymphatic vessels and it makes its way back up towards the heart where it drains into the vena cava and that fluid is returned to the cardiovascular system.
What happens if lymphatic system is overwhellmed with lymph fluid?
An accumulation of fluid known as oedema
What can cause oedema?
Raised CVP due to ventricular failure
Lymphatic obstruction due to surgery
Hypoproteinaemia due to liver failure
Increased capillary permeability due to inflammation
How does a raised central venous pressure cause left ventricular failure leading to odema?
If the left side of the heart isn’t pumping out blood effectively but the right side of the heart is still ok, you’re going to get blood accumulating in the lungs.
You’ll therefore get an increase in the hydrostatic pressure in the capillaries
and that’s going to lead to pulmonary oedema.
How does lymphatic blockage lead to odema?
A parasitic worm, a filarial worm that likes to live in the lymph nodes and when it does that, it blocks lymphatic drainage.
This therefore means that he has fluid accumulating in his legs and they are incredibly swollen, probably very sore.
Another cause of lymphatic obstruction is if you had surgery and it damaged some of the lymph nodes. Can cause unilateral oedema in this person’s left leg.
How can malnutrition cause odema?
Not enough protein in their diet.
Protein in the capillaries to build up that oncotic pressure and pull water back in.
Because they don’t have enough protein, they’re not building up that osmotic pressure and they’re therefore losing a lot more fluid into their lymphatic system.
What is the most common cause of oedema in the UK and what happens?
Hyperproteinaemia is due to nephrotic syndrome.
That’s where the kidneys are damaged and too much protein leaks out into the urine.
Hyperproteinaemia can also be the result of liver failure so if there’s problems in synthesising protein.
How does rhemuatoid arthiris cause odema?
Not enough protein in their capillaries to build up that osmotic pressure to pull water back in.
So they’re getting more fluid, being lost into their lymphatic system, and that’s leading to this oedema.
